The present invention relates to an amplifier system in general, and in particular to an amplifier system that can be operated in a linear output mode or a pulsed modulation output mode.
An amplifier system that drives a motor in high-precision industrial equipment should be accurate while maintaining a high level of energy transfer efficiency. Often high-precision industrial equipment, such as a lithography machine, has two modes of operation; a scanning mode and a stepping mode. In scanning mode, a positioning system positions a workpiece, such as a silicon wafer, in a precise and smooth manner. The workpiece is located on a stage. Thus, the amplifier system that drives a motor to move the stage in scanning mode should produce an accurate and linear output signal to the motor. Linear type amplifiers are therefore desirable for use in a high-precision positioning system while in scanning mode.
However, when the positioning system in the machine is in stepping mode, the workpiece on the stage is quickly moved from one position to the next. In stepping mode the positioning system generates large forces to accelerate and decelerate the stage to its new position. Thus, power transfer efficiency is important during stepping mode, and precision is less important. An amplifier system that produces a pulsed energy output, such as pulse width modulation (PWM) or pulsed period modulation (PPM), is highly efficient and therefore desirable for use in a positioning system while in stepping mode.
FIG. 1 is a schematic diagram of a dual stage amplifier system 10 used to produce a linear type output signal at terminal OUT as well as a PWM type output signal in response to an input signal at terminal IN. Amplifier system 10 includes a conventional linear amplifier output stage 20 with a PNP transistor 22 and a NPN transistor 24 arranged in a push-pull emitter follower configuration. Transistors 22 and 24 require resistors 25, 26, and diodes 27, 28 to eliminate crossover distortion.
Amplifier system 10 also has a PWM amplifier stage 30 including transistors 32 and 34. Transistors 32 and 34 in PWM amplifier stage 30 are turned off and transistors 22 and 24 are driven in their linear regions when a linear type output at terminal OUT is desired. When a PWM output is desired, transistors 22 and 24 in linear amplifier stage 20 are driven into saturation and the transistors 32 and 34 in PWM amplifier stage 30 are actuated. Thus, dual stage amplifier system 10 can be controlled to produce either linear type output signals or PWM type output signals.
The dual stage amplifier system 10 is disadvantageous because it requires two sets of transistors conducting simulatneously, resulting in decreased efficiency and precision. When amplifier system 10 is in PWM mode, the PWM output signal must be driven through saturated transistors 22 and 24, which generate heat. Thus, in PWM mode, transistors 22 and 24 generate heat. Moreover, transistors 22 and 24 require resistors 25, 26, and diodes 27, 28 to eliminate crossover distortion and, thus, further decrease efficiency.
The present invention is directed to a single stage amplifier system that produces an accurate linear output signal, i.e., the output signal is directly proportional to the input signal, during low power demands or when high performance is desired, and produces a pulse modulated output signal when power transfer efficiency is desired such as during high power demands. The amplifier system includes a control circuit that switches between a linear mode control circuit and a pulsed mode control circuit to control a single output stage. The single output stage includes two transistors in a push-pull arrangement that are driven in their saturation regions when a pulsed output signal is desired, and are driven within their linear region when a linear output signal is desired.
In one embodiment, the single stage amplifier system of the present invention is used in a half-bridge amplifier system to drive a load, such as a motor, while in another embodiment the signal stage amplifier system is used in an H-bridge amplifier system.